Heat-treated steel wire for high strength spring

ABSTRACT

The present invention provides a steel wire, for springs excellent in coiling property while having a high strength, as a heat treated steel wire for high strength springs, characterized by: comprising, in mass, C: 0.75 to 0.85%, Si: 1.5 to 2.5%, Mn: 0.5 to 1.0%, Cr: 0.3 to 1.0%, P: not more than 0.015%, S: not more than 0.015%, N: 0.001 to 0.007%, W: 0.05 to 0.3%, and the balance consisting of Fe and unavoidable impurities; having a tensile strength of not less than 2,000 MPa; spheroidal carbides, composed of mainly cementite, observed in a microscopic visual field satisfying the area percentage of the spheroidal carbides not less than 0.2 μm in circle equivalent diameter being not more than 7%, the density of the spheroidal carbides 0.2 to 3 μm in circle equivalent diameter being not more than 1 piece/μm 2 , and the density of the spheroidal carbides over 3 μm in circle equivalent diameter being not more than 0.001 piece/μm 2 ; the prior austenite grain size number being #10 or larger; the content of the retained austenite being not more than 12 mass %; the maximum diameter of carbides being not more than 15 μm; and the maximum diameter of oxides being not more than 15 μm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of PCT Application No.PCT/JP02/01049 which was filed on Feb. 7, 2002 and published on Aug. 5,2002as International Publication No. WO02/063055 (the “InternationalApplication”), the entire disclosure of which is incorporated herein byreference. This application claims priority from the InternationalApplication pursuant to 35 U.S.C. §365. The present application alsoclaims priority under 35 U.S.C. § 119 from Japanese Patent ApplicationNo. 2001-030511, filed on Feb. 7, 2001, the entire disclosure of whichis incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a steel wire for springs, which iscold-coiled and has high strength and high toughness.

BACKGROUND INFORMATION

Due to the trends toward weight reduction high performance ofautomobiles, springs have been strengthened and a high-strength steelhaving a tensile strength exceeding 1,500 MPa, after heat treatment, hasbeen applied to springs. In recent years, a steel wire having a tensilestrength exceeding 1,900 MPa has also been required. The purpose is tosecure a material hardness which does not cause problems when thematerial is used as a spring even though the material softens to someextent by heating in stress relief annealing, nitriding and the likewhen manufacturing a spring.

As a means to secure such a material, Japanese Unexamined PatentPublication No. S57-32353 discloses a method of generating finecarbides, which dissolve during quenching and precipitate duringtempering, by adding elements such as V, Nb, Mo, etc., and by so doing,controlling the movement of dislocations and thus improving settingresistance.

In the meantime, as methods to produce a steel coil spring, there arethe hot-coiling method wherein a steel is heated to a temperature in anaustenite region, coiled, and then quenched and tempered, and thecold-coiling method wherein a high-strength steel wire prepared bysubjecting a steel to quenching and tempering beforehand is coiled in acold state. In the cold-coiling method, since oil tempering treatment,high-frequency treatment or the like capable of employing rapid heatingand rapid cooling when producing a steel wire can be used, it ispossible to reduce the prior austenite grain size of a spring materialand, as a result, a spring excellent in fracture property can beproduced. Further, the method has an advantage of reducing the equipmentcost for a spring maker since an installation such as a heating furnacein a spring manufacturing line can be simplified, and therefore a shiftto the cold-coiling of a spring has advanced in recent years.

However, when the strength of a steel wire for a cold-coiled springincreases, it happens frequently that the steel wire breaks during thecold-coiling and cannot be formed into the shape of a spring. Therefore,there has been no other way than to coil a steel wire by a method whichcannot provide strength and workability simultaneously and seems to beindustrially disadvantageous. Usually, in the case of a valve spring, asteel wire after being subjected to quenching and tempering, namely oiltempering, on-line. is coiled. For example, in Japanese UnexaminedPatent Publication No. H05-179348, a wire is heated and coiled at atemperature where the wire is easily transformed during coiling toprevent breakage during the coiling in such a manner that a wire isheated to a temperature of 900 to 1,050° C. and coiled, and after thatis tempered at a temperature of 425 to 550° C., and thereafter the wireis subjected to conditioning treatment after the coiling to secure highstrength. Such heating during coiling and conditioning after the coilingcause the dispersion of spring dimensions after heat treatment or theradical deterioration of treatment efficiency, and therefore a springproduced by this method is inferior to a cold-coiled spring in both thecost and the dimensional accuracy.

With regard to the grain size of carbides, an invention developed bynoticing the average grain size of V or Nb system carbides is disclosedin Japanese Unexamined Patent Publication No. H10-251804, for example,and the invention shows that with only the control of the average grainsize of V or Nb carbides, sufficient strength and toughness cannot beobtained. Moreover, in this prior art, it is stated that there is aconcern that an abnormal structure appears, which is caused by coolingwater during rolling, and, therefore, it is recommended to substantiallyemploy dry rolling. From this description, it is assumed that the artinvolves unsteady work industrially and is apparently different fromusual rolling, and it suggests that even though the average grain sizeis controlled, troubles in rolling occur when the unevenness of a nearbymatrix structure is generated.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a steel wire, forsprings having a tensile strength of not less than 2,000 MPa, which iscoiled in a cold state and can secure both the sufficient strength inthe atmosphere and workability in the coiling, simultaneously.

The present inventors found that a steel wire for springs, which cansecure both the high strength and the coiling property simultaneously,can be obtained by controlling the size of carbides, particularlycementites, in steel, which had not been noticed in a conventional steelwire for springs.

The gist of the present invention is as follows:

(1) A heat treated steel wire for high strength springs, characterizedby:

comprising, in mass,

C: 0.75 to 0.85%,

Si: 1.5 to 2.5%,

Mn: 0.5 to 1.0%,

Cr: 0.3 to 1.0%,

P: not more than 0.015%,

S; not more than 0.015%,

N: 0.001 to 0.007%,

W: 0.05 to 0.3%, and

-   the balance consisting of Fe and unavoidable impurities;-   having a tensile strength of not less than 2,000 MPa;-   spheroidal carbides composed of mainly cementite, observed in a    microscopic visual field satisfying the ratio of the area occupied    by the spheroidal carbides not less than 0.2 μmin circle equivalent    diameter being not more than 7%, the density of the spheroidal    carbides 0.2 to 3 μm in circle equivalent diameter being not more    than 1 piece/μm², and the density of the spheroidal carbides over 3    μmin circle equivalent diameter being not more than 0.001 piece/μm    ²;-   the prior austenite grain size number being #10 or larger;-   the content of the retained austenite being not more than 12 mass %;-   the maximum diameter of carbides being not more than 15 μm; and-   the maximum diameter of oxides being not more than 15 μm.

(2) A heat treated steel wire for high strength springs according to theitem (1), characterized by further containing, in mass, one or two of

Mo: 0.05 to 0.2% and

V: 0.05 to 0.2%.

All referred-to references are incorporated herein by reference in theirentireties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph showing the quenched and tempered structureof a steel.

FIG. 2 consists of the graphs showing the examples of analyzingspheroidal carbides, (a) showing the example of analyzing alloy systemspheroidal carbides and (b) the same of analyzing spheroidal carbidescomposed of mainly cementite.

FIG. 3( a) is a schematic drawing showing the outline of the notchbending test method before loading.

FIG. 3( b) is a schematic drawing showing the outline of the notchbending test method after loading.

DETAILED DESCRIPTION

A steel wire capable of securing a coiling property sufficient formanufacturing springs by controlling the shape of carbides in steel witha heat treatment, according to the present invention is provided, whileregulating the chemical composition to obtain a high strength. Thedetails are provided herein below.

Firstly, the reasons for prescribing the chemical composition of steelwill be explained.

C is an element which greatly affects the basic strength of a steelmaterial, and is set at 0.75 to 0.85% so as to secure more strength thana conventional one. If less than 0.75%, a sufficient strength cannot beobtained. 0.75% or more of C is required for securing sufficient springstrength even when nitriding for spring performance improvement isexcluded, in particular. If C exceeds 0.85%, hyper-eutectoid appears andcoarse cementites precipitate in a large amount, and therefore thetoughness is deteriorated markedly. At the same time, this deterioratesthe coiling property too.

Si is an element necessary for securing sufficient strength, hardnessand setting resistance of a spring. If the amount is small, the strengthand setting resistance are insufficient and therefore the lower limit isset at 1.5%. Also, Si has the effect of spheroidizing and fining carbideprecipitates at grain boundaries, and by actively adding it, therearises the effect of decreasing the ratio of the area occupied by grainboundary precipitates in the grain boundaries. However, if Si is addedexcessively, the material not only hardens but also embrittles.Therefore, the upper limit is set at 2.5% for preventing theembrittlement after quenching and tempering.

The lower limit of Mn is set at 0.5% for securing sufficient hardnessand suppressing strength degradation by fixing S existing in steel asMnS. On the other hand, the upper limit is set at 1.0% for preventingthe embrittlement caused by Mn.

N hardens a steel matrix and, when an alloying element such as Ti or V,etc. is added, it exists as nitrides and affects the property of a steelwire. In a steel to which Ti, Nb, or V, etc. is added, carbonitrides areeasily generated and N is apt to form the sites where carbides, nitridesand carbonitrides which act as pinning particles for fining austenitegrains are precipitated. Thus, it is possible to stably generate pinningparticles under various heat treatment conditions employed during theproduction processes of springs and to control the austenite grain sizein a steel wire finely. For that purpose, 0.001% or more of N is added.On the other hand, N in excessive amount causes the coarsening ofnitrides and carbonitrides formed by the nitrides acting as nuclei andcarbides. For example, when Ti is added, coarse TiN precipitates, orwhen B is added, BN precipitates, and they cause deterioration offracture property. For those reasons, the upper limit of N is set at0.007% which does not cause problems.

P hardens a steel, and moreover generates segregation and thusembrittles a material. In particular, P segregating at austenite grainboundaries causes the deterioration of an impact value and delayedfracture caused by the intrusion of hydrogen. Therefore, the smallamount of P is preferable. For those reasons, P is restricted to notmore than 0.015% beyond which the embrittlement becomes remarkable. Salso embrittles a steel, as P does, when it exists in the steel. Thoughthe adverse effect can be alleviated by adding Mn, since MnS itselftakes the form of inclusions, the fracture property deteriorates. in thecase of a high-strength steel in particular, fracture occurs sometimescaused by a very small amount of MnS and therefore it is desirable tomake the S amount small. The upper limit of S is set at 0.015% beyondwhich the adverse effect becomes remarkable.

Cr is an element effective for improving quenching property andsoftening resistance in tempering. However, if the addition amount islarge, Cr not only increases the cost but also coarsens cementites whichappear after quenching and tempering. As a result, a wire becomesbrittle and thus breakage during coiling tends to occur. Therefore, thelower limit is set at 0.3% for securing a good quenching property and agood softening resistance in tempering, and the upper limit is set at1.0% beyond which the embrittlement becomes remarkable. In particular,when the amount of C is not less than 0.75% which is close to the rangeof eutectoid formation, it is better to suppress the amount of Cr forsuppressing the formation of coarse carbides and for securing both goodstrength and good coiling property simultaneously. On the other hand,when a nitriding treatment is employed, it is better to add Cr to makethe hardened layer formed by the nitriding deep. From the above, Cr isdetermined to be in the range of 0.3 to 1.0%.

W improves a quenching property and, at the same time, generatescarbides in a steel, and has the function to enhance strength.Therefore, it is preferable to add W as much as possible. The specificfeature of W is, different from other elements, to fine the shape ofcarbides including cementites. If the addition amount is less than0.05%, the effect does not appear, but if the same exceeds 0.3%, coarsecarbides are generated and there arises a concern of deterioratingmechanical properties such as ductility. For those reasons, the additionamount of W is set to be in the range of 0.05 to 0.3%.

Mo and V precipitate as nitrides, carbides and carbonitrides in a steel.Therefore, by adding one or two of these elements, it is possible toform those precipitates, obtain softening resistance in tempering, andthus demonstrate high strength without causing softening even after aheat treatment such as a tempering at a high temperature, a stressrelief annealing applied during processing, a nitriding and the like.This enables the deterioration of the hardness inside a spring after thenitriding to be suppressed, and the hot setting and the stress reliefannealing to be implemented easily, and therefore the fatigue propertyof the spring to improve as a whole. However, if the addition amount ofMo and v is too large, those precipitates grow too big, connect withcarbon in a steel, and generate coarse carbides. This causes the amountof C which should contribute to the high-strengthening of a steel wireto decrease and a strength equivalent to the amount of added C is notobtained. Moreover, since the coarse carbides become the source ofstress concentration, a steel wire tends to break due to the deformationduring coiling.

Mo can improve quenching property and secure softening resistance intempering by adding it at the percentage of 0.05 to 0.2. By so doing, itis possible to raise the tempering temperature when controllingstrength. This is advantageous in decreasing the ratio of grain boundaryarea occupied by grain boundary carbides. In other words, this iseffective for spheroidizing the grain boundary carbides precipitating inthe form of films by tempering them at a high temperature, and thusdecreasing the area ratio thereof in the grain boundaries. Further, Mogenerates, besides cementites, Mo system carbides in a steel. Inparticular, since Mo has a low precipitation temperature compared withV, etc., Mo shows the effect of suppressing the coarsening of carbides.The effect is not recognized when the addition amount is less than0.05%. However, if the addition amount is large, a supercoolingstructure tends to be generated during rolling, softening heat treatmentbefore drawing, or the like, and that is apt to cause cracks and wirebreakage during drawing. Therefore, when carrying out drawing, it ispreferable to draw a steel material after forming a ferrite-pearlitestructure in the steel material by a patenting treatment beforehand.However, when Mo exceeds 0.2%, the time up to the end of pearlitetransformation becomes long, the pearlite transformation cannot beterminated by a conventional patenting apparatus, and that causesmartensites to generate at the portions of micro-segregation which isunavoidably formed in a steel material. The martensites cause wirebreakage during drawing, or when they do not cause wire breakage andexist as internal cracks, they markedly deteriorate the properties ofthe final product. For these reasons, the upper limit is set at 0.2%wherein the generation of a martensite structure is suppressed androlling and drawing can be carried out easily and industrially stably.

With regard to V, it can be utilized for the hardening of a steel wireat a tempering temperature or the hardening of a surface layer duringnitriding, in addition to the suppressing of the coarsening of anaustenite grain size which is caused by the generation of nitrides,carbides and carbonitrides. When the addition amount is less than 0.05%,the effect of the addition is hardly recognized. On the other hand, theaddition in a large amount causes coarse insoluble inclusions to begenerated and toughness to deteriorate, and, at the same time, like Mo,a supercooling structure tends to be generated and that is apt to causecracks and wire breakage during drawing. For those reasons, the upperlimit is set at 0.2% wherein an industrially stable operation can becarried out easily. The prescription of the carbides will be explainedhereunder. To obtain both strength and workability simultaneously, theconfiguration of carbides in a steel is important. Here, the carbides ina steel means: the cementites generated after heat treatment and thecarbides formed by dissolving alloying elements therein (both arehereunder referred to as “cementites” in general); and the carbides andcarbonitrides of alloying elements such as Nb, V, Ti, etc. Thosecarbides can be observed by specularly polishing and etching a steelwire.

A typical example of the observation is shown in FIG. 1. According tothe photomicrograph, observed are the two kinds of carbides, acicularand spheroidal ones. In general, it is known that a steel forms acicularstructures composed of martensites by quenching and generates carbidesby tempering, and, by so doing, both strength and toughness can beobtained simultaneously. However, the present inventors noticed that notonly acicular structures but also spheroidal carbides 1 remained in agreat quantity as shown in FIG. 1, and found that the distribution ofthe spheroidal carbides greatly affected the properties of a steel wirefor springs. It is estimated that the spheroidal carbides are carbideswhich are not sufficiently dissolved by the quenching and tempering inan oil-tempering treatment or a high frequency treatment and arespheroidized and grow or shrink in the quenching and temperingprocesses. The carbides of this size do not contribute at all to theimprovement of strength and toughness by quenching and tempering. Basedon that, the present inventors found that the spheroidal carbides notonly wasted the added C by fixing C in a steel but also acted as thesource of stress concentration, and thus became a factor indeteriorating the mechanical properties of a steel wire.

In the case of cold-coiling a steel after quenching and tempering thesteel as seen in this material, carbides affect the coiling property,namely the bending property until breakage occurs. Though it has beengenerally adopted up to now to add in a great quantity not only C butalso alloying elements such as Cr, V, etc. for obtaining strength, therehave been drawbacks of too high a strength, insufficient deformationcapability and a deteriorated coiling property. It is estimated that thedrawbacks are caused by the coarse carbides precipitating in a steel.

Examples of the analysis using an energy dispersion X-ray analyzer (EDX)attached to a scanning electron microscope (SEM) are shown in (a) and(b) of FIG. 2. Analysis results similar to those results are alsoobtained by the replica method using a transmission electron microscope.Conventional inventions pay attention to only the carbides of alloyingelements such as V, Nb, etc. and an example thereof is shown in FIG. 2(a), which is characterized in that the peak of Fe in the carbides isextremely small. On the other hand, in the present invention, it wasfound that, not only the conventional alloying element carbides but alsothe configuration of the precipitation of what is called carbides,composed of mainly cementite, which were composed of Fe₃C 3 μm or lessin circle equivalent diameter and alloying elements scarcely dissolvedtherein as shown in FIG. 2( b), is important. In the case of attainingsimultaneously both high strength and workability more excellent thanthose of conventional steel wires, as in the case of the presentinvention, if the spheroidal carbides, composed of mainly cementite, 3μm or less in circle equivalent diameter are abundant, the workabilityis markedly deteriorated. Hereafter, the spheroidal carbides mainlycomposed of Fe and C as shown in FIG. 2( b) are referred to as “carbidescomposed of mainly cementite.”

Those carbides in a steel can be observed by applying an etchingsolution such as picral to a test piece specularly polished. However, inorder to observe and evaluate their dimensions and the like further indetail, it is necessary to observe them at a high magnification over3,000 times using a scanning electron microscope. The size of thespheroidal carbides, composed of mainly cementite, discussed here is 0.2to 3 μm in circle equivalent diameter. Usually, carbides in a steel isessential for securing the strength and softening resistance in thetempering of the steel, but the effective grain size is not more than0.1 μm, and if it exceeds 1 μm, on the contrary, the carbides do notrather contribute to the fining of an austenite grain size and merelydeteriorate the deformation property. However, in the prior art, theimportance was not well recognized, only the carbides of the systemcontaining the alloying elements such as V, Nb, etc. were noticed, thecarbides 3 μm or less in circle equivalent diameter, spheroidal carbidescomposed of mainly cementite in particular, were regarded to beharmless, and therefore an instance wherein the carbides having a sizeof about 0.1 to 5 μm, which are the major objects of the presentinvention, were studied cannot be found.

Further, in the case of the spheroidal carbides composed of mainlycementite 3 μm or less in circle equivalent diameter which are theobjects of the present invention, not only the size but also the numberis a large factor. Therefore, the scope of the present invention isprescribed taking both factors into consideration. That is, even thoughthe circle equivalent diameter is small in the range of 0.2 to 3 μm inaverage diameter, when the number is very large and the density in amicroscopic visual field exceeds 1 piece/μm², then the coiling propertyremarkably deteriorates and therefore the upper limit is set at 1piece/μm².

Further, if the size of the carbides exceeds 3 μm, the influence of thesize becomes further remarkable, and in that situation, if the densityin a microscopic visual field exceeds 0.001 piece/μm², then the coilingproperty remarkably deteriorates. Therefore the upper limit of thedensity of the carbides over 3 μm in circle equivalent diameter in amicroscopic visual field is set at 0.001 piece/μm², and the range in thepresent invention is set at not more than that value.

Further, disregarding the size of the spheroidal carbides composed ofmainly cementite, if the area percentage of the spheroidal carbides in amicroscopic visual field exceeds 7%, then the coiling propertyremarkably deteriorates and coiling operation becomes impossible.Therefore, the area percentage thereof in a microscopic visual field isset at not more than 7%.

However, a prior austenite grain size, similar to a carbide grain size,exerts a great influence on the fundamental properties of a steel wire.More specifically, the smaller the prior austenite grain size is, themore excellent the fatigue property and coiling property are. However,however small the prior austenite grain size may be, the effect is smallif the above-mentioned carbides are abundantly contained and exceed theprescription. It is effective in general to lower a heating temperatureto reduce the austenite grain size, but, on the contrary, this causesthe above-mentioned carbides to increase. Therefore, it is important tofinish a steel wire so that the carbide amount and the prior austenitegrain size are appropriately balanced. In this connection, on thepremise that the carbides satisfy the above prescription, the prioraustenite grain size number is prescribed to be not less than #10,because, if the prior austenite grain size number is less than #10,sufficient fatigue property cannot be obtained.

Retained austenites tend to remain in the vicinity of segregatedportions and prior austenite grain boundaries. It was found that, thoughthe retained austenites transformed into martensites by work inducedtransformation, if the induced transformation occurred during springforming, highly hardened portions were locally generated in the materialand the coiling property of a spring was rather deteriorated. Recently,springs have been subjected to surface strengthening by applying plasticdeformation such as shot peening or setting and, in the case wheremanufacturing processes including plural processes wherein such plasticdeformation is applied, are employed, the work induced martensitesgenerated in an early stage lower the fracture strain and deterioratethe workability and fracture property of springs in service. Further, inthe case where industrially unavoidable deformations such as dents andthe like are present, a steel wire easily breaks during coiling.Therefore, the workability is improved by reducing retained austenitesto the utmost and suppressing the generation of work inducedmartensites. Concretely, if the amount of retained austenites exceeds12% (in weight), the susceptibility to dents and the like increases andbreakage easily occurs during coiling and other operations. Therefore,the amount of retained austenites is set at not more than 12%.

In particular, in the case where the amount of C is not more than 0.75%as the case of the present invention, if the martensite generatingtemperature (start temperature: Ms point, finish temperature: Mf point)becomes low, martensites are not generated and retained austenites areapt to remain unless the temperature during quenching is loweredsufficiently. Water or oil is used for quenching industrially, but asophisticated heat treatment control is required for suppressingretained austenites. More specifically, required is an appropriatecontrol such as to keep the temperature of a coolant low, to keep thetemperature low to the utmost even after the cooling, to keep the timeof transformation to martensites long, or the like. Though thetemperature of a coolant easily rises close to 100° C. industrially, asthe treatments are carried out in a continuous line, it is preferable tokeep the temperature thereof to not more than 60° C.

Further, when both the maximum grain size of all carbides includingalloying element carbides and the like and the maximum grain size ofoxides exceed 15 μm, that causes the fatigue property to deteriorate.Therefore, the upper limits of the maximum grain sizes thereof are setat 15 μm, respectively.

In general, a steel for springs is, after being continuously cast,rolled into billets, rolled into wire rods and then drawn into wires,and after that, in the case of cold-coiled springs, the drawn wires aregiven strength by applying an oil temper treatment or a high frequencytreatment. For suppressing the spheroidal carbides composed of mainlycementite, it is necessary to pay attention not only to the final heattreatment such as an oil temper treatment, a high frequency treatment orthe like, which determines the strength of a steel wire, but also to therolling processes which precede the drawing process. In other words, asit is considered that the spheroidal carbides composed of mainlycementite grow with cementites and alloyed carbides insoluble during therolling processes and the like acting as nuclei, it is important tofully dissolve the components during each heating process in rolling. Inthe present invention, it is important to heat a steel material to asufficiently high temperature, even in the rolling processes, then rollit, and draw it.

EXAMPLE

Table 1 shows, in the case of the steel wires 4 mm in diameter and withregard to Invented Examples and Comparative Examples: chemicalcompositions; the ratios of the areas occupied by spheroidal carbidescomposed of mainly cementite 0.2 μm or more in circle equivalentdiameter; the densities of spheroidal carbides composed of mainlycementite 0.2 to 3 μm in circle equivalent diameter; the densities ofspheroidal carbides composed of mainly cementite over 3 μm in circleequivalent diameter; the maximum diameters of carbides and oxides; prioraustenite grain size numbers; the amounts of retained austenites (inweight %); tensile strength; coiling property (in terms of notch bendingangle); and average fatigue strength.

In Invented Example 1 according to the present is invention, a billetwas produced by continuously casting steel refined with a 250 tonconverter. In the other Invented Examples and all Comparative Examples,billets were produced by rolling after steel was melted and refined witha 2 ton vacuum melting furnace. In those cases, Invented Examples wereretained at a high temperature of not less than 1,200° C. for aprescribed period of time. After that, in all cases, the billets wererolled into wire rods 8 mm in diameter, and then steel wires 4 mm indiameter were prepared by drawing. In the case of Comparative Examples,the billets were rolled under the usual conditions and drawn.

Since the amount of carbides and strength vary depending on the chemicalcompositions, in the Invented Examples, the materials were heat-treatedin conformity with the chemical compositions so as to secure the tensilestrength of about 2,100 MPa and satisfy the prescriptions shown in theclaims. On the other hand, in Comparative Examples, the materials wereheat-treated merely so as to equalize the tensile strength.

In the quenching and tempering treatment (oil tempering treatment), thedrawn materials were passed through a heating furnace continuously andthe time required for passing through the heating furnace was determinedso that the interior of the steel was sufficiently heated. In bothInvented Examples and Comparative Examples, heating temperature was setat 950° C., heating time at 150 sec., and quenching temperature at 50°C. (in an oil tank). After that, the materials were tempered at atempering temperature of 400 to 500° C. for 1 min. of tempering time,and the strength was adjusted. The resultant tensile strength in theatmosphere is listed in Table 1.

TABLE 1 Fatigue Maximum Prior Bending strength in diameter Maximumaustenite angle rotating of diameter grain Tensile in bending Chemicalcompositions Area Density carbide of oxide size Retained strength notch-fatigue Example No. C Si Mn P S Cr W V Mo N percentage % 0.2–3 >3 μm μmnumber austenite % MPa bending MPa Inventive 1 0.84 1.97 0.92 0.0080.007 0.47 0.22 0.0045 2.5 0.15 <0.0001 12.2 11.0 12 8.0 2097 36 867Steel Inventive 2 0.79 1.74 0.97 0.008 0.011 0.35 0.19 0.0054 0.6 0.03<0.0001 10.6 11.4 13 7.1 2106 38 854 Steel Inventive 3 0.77 1.84 0.840.010 0.003 0.50 0.13 0.0051 0.5 0.21 <0.0001 10.5 10.9 11 9.7 2093 38855 Steel Inventive 4 0.79 1.70 0.91 0.006 0.006 0.38 0.09 0.11 0.00511.5 0.09 <0.0001 12.4 11.5 11 10.9 2074 36 857 Steel Inventive 5 0.831.71 0.66 0.003 0.006 0.31 0.27 0.17 0.0021 1.7 0.27 <0.0001 11.1 11.410 11.5 2176 33 888 Steel Inventive 6 0.75 1.91 0.56 0.010 0.005 0.340.19 0.21 0.0034 1.5 0.23 <0.0001 10.1 10.0 11 10.3 2089 39 855 SteelInventive 7 0.81 1.91 0.91 0.009 0.008 0.35 0.14 0.16 0.20 0.0038 1.90.31 <0.0001 12.9 11.4 12 8.6 2141 33 869 Steel Inventive 8 0.80 2.000.88 0.005 0.007 0.37 0.06 0.0053 1.2 0.16 <0.0001 11.2 11.5 12 10.52143 37 861 Steel Inventive 9 0.82 1.69 0.70 0.007 0.006 0.38 0.12 0.180.0022 0.2 0.15 <0.0001 11.4 12.7 12 10.5 2102 36 865 Steel Inventive 100.76 1.86 0.95 0.004 0.011 0.42 0.23 0.24 0.07 0.0024 1.5 0.01 <0.000112.9 12.2 12 11.2 2158 42 854 Steel Inventive 11 0.81 1.86 0.95 0.0070.005 0.34 0.10 0.25 0.17 0.0053 1.2 0.09 <0.0001 12.3 10.6 12 9.4 210936 855 Steel Inventive 12 0.79 1.86 0.80 0.006 0.006 0.49 0.16 0.18 0.150.0025 0.1 0.15 <0.0001 10.7 12.7 12 11.0 2184 40 887 Steel Comparative13 0.81 1.64 0.92 0.007 0.008 1.45 0.46 0.21 0.0041 8.5 0.62 <0.000110.5 11.0 13 10.8 2165 16 876 Steel Comparative 14 0.84 1.81 0.78 0.0120.012 1.65 0.29 0.16 0.0021 9.1 1.25 <0.0001 11.4 11.2 11 9.7 2116 17888 Steel Comparative 15 0.82 1.99 0.81 0.005 0.003 1.52 0.43 0.120.0046 2.9 1.65 <0.0001 11.2 10.1 11 8.9 2137 21 850 Steel Comparative16 0.92 1.78 0.73 0.005 0.012 0.78 0.21 0.18 0.0051 1.9 0.02 0.003 10.610.1 12 8.5 2138 37 788 Steel Comparative 17 0.64 1.56 0.96 0.004 0.0100.85 0.14 0.0047 0.8 0.23 <0.0001 11.1 12.8 11 8.4 1892 31 772 SteelComparative 18 0.91 1.79 0.50 0.006 0.007 0.91 0.13 0.0055 5.7 1.35<0.0001 22.0 11.5 12 7.5 2123 16 871 Steel Comparative 19 0.92 1.72 0.700.009 0.008 0.92 0.11 0.0054 1.3 0.25 <0.0001 11.9 24.0 11 10.1 2101 21815 Steel Comparative 20 0.85 1.57 0.76 0.004 0.003 0.64 0.12 0.53 0.730.0023 2.9 0.31 <0.0001 10.0 10.2 13 13.2 2209 18 796 Steel Comparative21 0.75 1.92 0.79 0.007 0.009 0.88 0.05 0.49 0.64 0.0055 6.5 0.91<0.0001 30.0 10.4 12 7.1 2176 16 821 Steel Comparative 22 0.75 1.91 0.830.008 0.010 0.88 0.05 0.54 0.65 0.0051 5.5 1.21 <0.0001 10.4 12.2 9 9.02200 17 876 Steel Comparative 23 0.84 1.77 0.99 0.004 0.012 0.99 0.060.32 0.60 0.0051 9.3 0.05 <0.0001 12.7 10.8 10 11.3 2158 21 789 Steel

The steel wires thus produced were subjected directly to the evaluationof carbides, the tensile strength test and the notch bending test. Withregard to the fatigue property evaluation, the test pieces for fatiguetest were prepared by: applying the heat treatment at 400° C. for 20min. to the surfaces of the steel wires, simulating the stress reliefannealing in the actual production of springs; after that, applying theshot peening treatment (cut wires 0.6 mm in diameter, for 20 min.); andthen applying another stress relief annealing at a low temperature of180° C. for 20 min.

The evaluation of the size and number of carbides was carried out byspecularly polishing the cross section, in the longitudinal direction,of the steel wires directly after being heat-treated, slightly etchingthe polished surfaces with picric acid, and embossing carbides. Sincethe measurement of the size of carbides with a means having the accuracyof an optical microscope was difficult, a scanning electron microscopewas used and the photographs of the ½ R portions of the steel wires weretaken from ten visual fields at random at a magnification of 5,000. Thesize, the number and the ratio of occupied area of each test piece weremeasured by binary coding the spheroidal carbides applying an imageprocessing apparatus to the photograph, while confirming that thespheroidal carbides are really the cementite system spheroidal carbidesusing an x-ray microanalyzer attached to a scanning electron microscope.The whole measured area was 3,088.8 μm².

The amount of retained austenites was obtained by measuring the magneticflux density of each test piece generated using a direct currentmagnetization apparatus and converting the magnetic flux density intothe amount of retained austenites. For the conversion, a calibrationcurve, prepared beforehand by specifying the relation between themagnetic flux density and the amount of retained austenites, was used.

As for the tensile property, tensile strength was measured by conductingthe test according to JIS (Japanese Industrial Standards) Z 2241 using atest piece of No. 9 defined in JIS Z 2201, and being calculated from thebreaking load obtained.

The outline of the notch bending test is shown in (a) and (b) of FIG. 3.The notch bending test was conducted, in order, by: forming a groove(notch) 30 μm in maximum depth perpendicularly to the longitudinaldirection of a steel wire with a punch having a tip 50 μm in radius;imposing a bending deformation on the groove at the three points with aload 2 so that the maximum tensile stress was imposed on the groove asshown in FIG. 3( a); continuing to impose the bending deformation untilthe steel wire broke at the notched portion; and measuring the bendingangle when the breakage occurred as shown in FIG. 3( b). A measuredangle 3 is as shown in FIG. 3( b). The larger the angle is, the betterthe coiling property is. Empirically, if a notch bending angle is notmore than 25 degree in the case of a steel wire 4 mm in diameter, thesteel wire can hardly be coiled.

For the fatigue test, Nakamura's rotating bending fatigue test wasemployed, and a maximum load stress where 10 test pieces showed the lifeof not less than 10⁷ cycles at the probability of not less than 50% wasdetermined as an average fatigue strength.

As shown in Table 1, in the case of the steel wires 4 mm in diameter,when the chemical composition of a steel wire is outside the range ofthe prescription, the control of carbides is hardly implemented, thebending angle in the notch bending test, which acts as the index of thecoiling property, becomes small, thus the coiling property deteriorates,and the fatigue strength in the Nakamura's rotating bending fatigue testalso deteriorates. Further, even though the chemical compositions ofsteel wires are within the range of the prescription, in the case ofComparative Examples wherein the maximum diameter of oxides and thegrain diameter of prior austenites are outside the range of theprescription according to the present invention, caused by theinappropriate heat treatment conditions, such as the remaining ofinsoluble carbides caused by the insufficient heating during thestabilizing of carbides in prior annealing or during the quenching,insufficient cooling during the quenching, and the like, the coilingproperty or the fatigue strength deteriorates. On the other hand, eventhough the prescription related to carbides is satisfied, when thestrength is insufficient, fatigue strength is also insufficient and thussuch a steel wire cannot be used for high strength springs.

INDUSTRIAL APPLICABILITY

A steel wire according to the present invention can have a high strengthof not less than 2,000 MPa and enables high strength springs excellentin fatigue property to be produced while securing the coiling propertyby means of reducing the ratio of the area occupied by spheroidalcarbides including cementites, the density of the spheroidal carbides,the austenite grain size, and the amount of retained austenites in thesteel wire for cold-coiling springs.

1. A heat treated steel wire provided for at least one high strengthspring, characterized by: comprising, in mass, C: 0.75% to 0.85%, Si:1.5% to 2.5%, Mn: 0.5% to 1.0%, Cr: 0.3% to 1.0%, P: not more than0.015%, S: not more than 0.015%, N: 0.001% to 0.007%, W: 0.05% to 0.3%,and the balance consisting of Fe and unavoidable impurities, wherein atensile strength of the steel wire is at least 2,000 MPa, whereinspheroidal carbides of the steel wire are composed of mainly cementiteobserved in a microscopic visual field, the cementite satisfying thearea percentage of the spheroidal carbides of at least 0.2 μm in acircle equivalent diameter being not more than 7%, the density of thespheroidal carbides 0.2 μm to 3 μm in circle equivalent diameter beingat most 1 piece/me, a density of the spheroidal carbides being over 3 μmin circle equivalent diameter being not more than 0.001 piece/me,wherein a prior austenite grain size number of the steel wire is atleast #10, wherein the content of a retained austenite of the steel wireis at most 12 mass %, wherein a maximum diameter of the spheroidalcarbides is at most 15 μm, and wherein a maximum diameter of oxides ofthe steel wire at most 15 μm.
 2. The heat treated steel wire accordingto claim 1, further characterized by: comprising, in mass, one or twoof: Mo: 0.05% to 0.2% and V: 0.05% to 0.2%.